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Gọi vận tốc dự định đi quãng đường AB = x km/giờ (x>0)
thời gian dự định đi quãng đường AB = 60/x
--> vận tốc đi nửa đường đầu (đi 30km đầu) = x + 10
thời gian đi nửa đường đầu = 30/(x + 10)
--> vận tốc đi nửa đường sau (đi 30 km sau) = x - 6
thời gian đi nửa đường sau = 30/(x -6)
Theo đề ta có
30/(x + 10) + 30/(x - 6) = 60/x
1/(x + 10) + 1/(x - 6) = 2/x
x(x-6) + x(x+10) = 2(x-6)
(x+10)x^2 - 6x + x^2 +10x = 2(x^2 + 4x - 60)
2x^2 + 4x = 2x^2 + 8x - 120
4x = 120
x = 30 (thỏa mãn)
Vậy thời gian dự định đi quãng đường AB là t = 60/x = 60/30 = 2 giờ
gọi x(km) là quãng đường xe đi từ A đến B ( x∈N)
Thời gian xe đi từ a đến B là theo dự định: \(\frac{x}{50}\)
Thờ gian xe đi từ A đến B do thờ tiết xấu là : \(\frac{x}{40}\)
mà do thời tiết xấu nên xe đến chậm 1/2h
⇒ \(\frac{x}{40}-\frac{x}{50}=\frac{1}{2}\)
⇔50x-40x=100
⇔ 10x = 100
⇔ x = 10
Vậy quãng đường AB là 10km
1.Nga sometimes goes to her friend,s at the weekend.
2.We are going to Nha Trang this Sunday
3.Where are your children at the moment?
5.We eating some fruits.
1.Nga sometimes goes to her friend,s at the weekend.
2.We are going to Nha Trang this Sunday
3.Where are your children at the moment?
5.We eating some fruits.
* Exercise. Fill in each space with a correct preposition
1. Nam is never late..for.....school.
2. They are waiting..for. Marry ..at..the bus stop at the moment
3. We often study English..on...Monday, Tuesday and Saturday.
4. She built this house ..in. 1998
5. My father often goes to work .at..7 o'clock
6. He will come .between...6.30 and 7.00
7. Mo will stay with us..from..Monday to Friday
8. He arrived..in..Singapore..on.....Monday evening.
9. Goodbye! See you..on...Sunday
10. The bus collected us .at..5 o'clook early..in.the morning
11. We usually go to our home village at least once.in..the summer
12. They planned to have a trip.in..June
13. She loves to watch the stars.at..night
14.Nam never travels to work ..by.....car
16. I am very proud ....of...our school
15. My mother went to work..on.foot last Sunday
17. She is fond..of....watching T.V
18. He is afraid.of..living alone in a big city
19. Mo is angry..at.me because I am angry..with...her dog
20. Son is interested..in....reading novel
21. Ha Noi is famous ...for.... its beauty and its people
23. Nam is fed up.....with...doing the same things everyday
22. How...about..watching T.V at my home this evenng?
24. She is bored.with....eating the same thing everyday.
25. It is not convenient..for.you to call someone at night.
26. Milk is very good..for..our health
27. Ha Noi is very different..from..Paris
28. The hall is full..of.students waiting.for...their teacher.
29. He is always short.of..money.
30. They are worried...about...missing the train.
31. They left the restaurant..without..paying the bill.
32. ..in..National day, people usually walk in large groups.
33. I am surprised..at.his success
34. Thank you very much...fot...lending me your money.
35. Alexander Graham was born.in...Edinburgh .in...March, 1847
36. You should turn..off... all the lights before going out.
37. He has to stay at...home to look..after..his baby because his mother went out
38. She is looking .for..her lost pen.
39. I will see you...off...at the airport tomorrow.
40. If you don't pay the electricity bill,the electricity will be cut..off...
41. My mother often buys presents...for.me on my birthday
42. We have to put .on...our shoes before going in the pagoda.
43. My shoe is very old. It was worn out
44. My father usually helps me ...with..my homework.
45. I got up late because my arlam clock didn't go..off....
46. This table was made..of..wood
47. Don't look .at..me
48. Mo is very good...at...English
49. My car is still...in..good condition
50. He is always.on..time.